Grease a 9-by-13-inch baking pan, then line with enough parchment paper to leave several inches of overhang on each long side.
In a large, heavy saucepan, melt 1 stick butter over medium-low heat. Stir in 2 tablespoons molasses, 1 1/2 tablespoons pumpkin pie spice and 1 teaspoon salt. Stir in 2 bags of mini marshmallows and cook over medium-low heat, stirring occasionally, until the marshmallows are melted. Meanwhile, pour 12 cups cereal into a large bowl. Add the cereal to the marshmallow mixture and fold in quickly to coat. Scrape the cereal mixture into the prepared pan; let cool for a few minutes.
Lightly coat your hands with oil, then press the cereal mixture evenly into the pan. Using a dry measuring cup, pack down the mixture to even out the layer. Let cool for 30 minutes. Loosen the edges with a knife, then grasp the parchment and lift to remove the cooled cereal layer from the pan. Repeat Steps 1, 2 and 3 to make 2 more cereal layers.
Assemble the house and decorate with the remaining ingredients.